The present invention relates to a new process for the production of composite bodies of polyurethane foams and covering layers, by placing a covering layer inside a mold, and filling the mold with a reaction mixture which reacts to form polyurethane foam. The invention also relates to the composite bodies produced according to this process.
The production of composite bodies by foaming polyurethane in contact with covering layers is known in principle. It is generally carried out by foam molding a reactive and foamable mixture of organic polyisocyanates, compounds containing isocyanate reactive groups and the usual auxiliary agents and additives, the covering layer having previously been placed inside the mold and suitably fixed in position, i.e. the internal wall of the mold having preferably been at least partly lined with the covering layer.
Both flexible and semi-rigid and rigid moldings may be produced by this process with the appropriate choice of the starting components. In particular, the molecular weight and functionality of the starting components should be considered.
One fundamental problem in the production of such composite bodies is the adherence between the reaction mixture which reacts to form the polyurethane foam and the covering layer. Important advances have been achieved in this respect, for example, by providing suitable bonding agents. Thus, for example, the adherence of mixtures which react to form polyurethane to metals has been substantially improved by pretreating the metal surface with aqueous polyurethane dispersions (DE-AS 2,633,764).
New problems in the adherence of the polyurethane foams to covering layers have arisen when CFC blowing agents for polyurethane foams have been replaced by water/carbon dioxide as blowing agents. The polyurethane foams thus produced adhere much less firmly to the materials conventionally used as covering layers. It has now been found that a marked improvement in the adherence of foams produced with water as a blowing agent can be achieved by the addition of salts containing water of crystallization, preferably salts which have a solubility below 5 g/l in water at 20.degree. C.
The use of salts containing water of crystallization as additives for the production of polyurethane resins is known. However, the relevant state of the art gives no hint of the effect of these additives in improving the adherence of polyurethane foams to the covering layer.
GB-PS 1,147,695 describes the use of salts containing water of crystallization for the preparation of polyurethanes which are not foamed. DE-OS 1,806,404 describes the use of salts containing water of crystallization for the production of foam molded articles which have particularly uniform properties. The same applies to DE-OS 2,651,400, which relates to the use of compounds containing water of crystalization in one-component systems. None of these references disclose any information concerning the above-identified problem of foam adherence to the covering layers.
According to an earlier proposal of our own (German Patent Application P 40 13 141.6), salts containing water of 25, crystallization are used as blowing agents for the production of molded foams having a compact surface. These products are produced without any covering layers.